Add Material.get_nuclide_activity method and decay_constant function

docs/source/pythonapi/data.rst

@@ -61,6 +61,7 @@ Core Functions
 
     atomic_mass
     atomic_weight
+    decay_constant
     dose_coefficients
     gnd_name
     half_life

openmc/data/data.py

@@ -3,7 +3,7 @@ import json
 import os
 import re
 from pathlib import Path
-from math import sqrt
+from math import sqrt, log
 from warnings import warn
 
 # Isotopic abundances from Meija J, Coplen T B, et al, "Isotopic compositions
@@ -202,7 +202,7 @@ _GND_NAME_RE = re.compile(r'([A-Zn][a-z]*)(\d+)((?:_[em]\d+)?)')
 
 # Used in half_life function as a cache
 _HALF_LIFE = {}
-
+_LOG_TWO = log(2.0)
 
 def atomic_mass(isotope):
     """Return atomic mass of isotope in atomic mass units.
@@ -283,6 +283,8 @@ def half_life(isotope):
     Half-life values are from the `ENDF/B-VIII.0 decay sublibrary
     <https://www.nndc.bnl.gov/endf-b8.0/download.html>`_.
 
+    .. versionadded:: 0.13.1
+
     Parameters
     ----------
     isotope : str
@@ -302,6 +304,35 @@ def half_life(isotope):
 
     return _HALF_LIFE.get(isotope.lower())
 
+
+def decay_constant(isotope):
+    """Return decay constant of isotope in [s^-1]
+
+    Decay constants are based on half-life values from the
+    :func:`~openmc.data.half_life` function. When the isotope is stable, a decay
+    constant of zero is returned.
+
+    .. versionadded:: 0.13.1
+
+    Parameters
+    ----------
+    isotope : str
+        Name of isotope, e.g., 'Pu239'
+
+    Returns
+    -------
+    float
+        Decay constant of isotope in [s^-1]
+
+    See also
+    --------
+    openmc.data.half_life
+
+    """
+    t = half_life(isotope)
+    return _LOG_TWO / t if t else 0.0
+
+
 def water_density(temperature, pressure=0.1013):
     """Return the density of liquid water at a given temperature and pressure.
 

openmc/material.py

@@ -3,7 +3,6 @@ from collections.abc import Iterable
 from copy import deepcopy
 from numbers import Real
 from pathlib import Path
-import math
 import re
 import warnings
 from xml.etree import ElementTree as ET
@@ -145,20 +144,10 @@ class Material(IDManagerMixin):
 
         return string
 
-
     @property
     def activity(self):
         """Returns the total activity of the material in Becquerels."""
-
-        atoms_per_barn_cm = self.get_nuclide_atom_densities()
-        total_activity = 0
-        for key, value in atoms_per_barn_cm.items():
-            half_life = openmc.data.half_life(key)
-            if half_life:
-                total_activity += value / half_life
-        total_activity *= math.log(2) * 1e24 * self.volume
-
-        return total_activity
+        return sum(self.get_nuclide_activity().values())
 
     @property
     def name(self):
@@ -861,6 +850,23 @@ class Material(IDManagerMixin):
 
         return nuclides
 
+    def get_nuclide_activity(self):
+        """Return activity in [Bq] for each nuclide in the material
+
+        .. versionadded:: 0.13.1
+
+        Returns
+        -------
+        dict
+            Dictionary whose keys are nuclide names and values are activity in
+            [Bq].
+        """
+        activity = {}
+        for nuclide, atoms in self.get_nuclide_atoms().items():
+            inv_seconds = openmc.data.decay_constant(nuclide)
+            activity[nuclide] = inv_seconds * atoms
+        return activity
+
     def get_nuclide_atoms(self):
         """Return number of atoms of each nuclide in the material
 

tests/unit_tests/test_data_misc.py

@@ -1,6 +1,6 @@
 #!/usr/bin/env python
 
-from collections.abc import Mapping
+from math import log
 import os
 from pathlib import Path
 
@@ -126,3 +126,14 @@ def test_zam():
     assert openmc.data.zam('Am242_m10') == (95, 242, 10)
     with pytest.raises(ValueError):
         openmc.data.zam('garbage')
+
+
+def test_half_life():
+    assert openmc.data.half_life('H2') is None
+    assert openmc.data.half_life('U235') == pytest.approx(2.22102e16)
+    assert openmc.data.half_life('Am242') == pytest.approx(57672.0)
+    assert openmc.data.half_life('Am242_m1') == pytest.approx(4449622000.0)
+    assert openmc.data.decay_constant('H2') == 0.0
+    assert openmc.data.decay_constant('U235') == pytest.approx(log(2.0)/2.22102e16)
+    assert openmc.data.decay_constant('Am242') == pytest.approx(log(2.0)/57672.0)
+    assert openmc.data.decay_constant('Am242_m1') == pytest.approx(log(2.0)/4449622000.0)